BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI

68 Bogdan Horbaniuc et al
1. Introduction
Thermal energy storage is a technique used to match the energy supply
(such as wind energy, solar energy, etc.) and the consumer’s energy demand
because the former exhibits important fluctuations or cut-offs (periodical or
random).
Thermal energy storage (TES) is of two types: sensible heat TES (heat is
stored as specific heat of the sensible storage material (SSM) – water, rock, soil,
aquifers, etc.), respectively latent heat TES (heat is stored as latent heat of
fusion of a substance that undergoes a phase transition – molten salts, paraffins,
alloys, etc.).
The immediate benefits of TES (Dincer & Rosen, 2011) derive mainly
from the increase of generation capacity by a better match of the supply and
demand energy curves (because very seldom a peak in demand can be
synchronized with primary energy supply), and from the possibility to shift
energy consumption to low-cost periods.
The most important advantages of TES are (Dincer & Rosen, 2001),
(Dincer, 2002): reduced energy costs and consumption, flexibility in operation,
reduced operation costs, increased efficiency of equipment utilization, and
reduced pollutant emissions and thus a reduced ecological impact.
The performance of a TES can be assessed by means of parameters such
as: storage capacity [kWh/m 3 ], storage density [kWh/m 3 , or kWh/kg],
charge/discharge rates [kW], storage efficiency determined by means of heat
losses [kW], and SSM thermal cycling (the capacity of the SSM to preserve its
thermophysical properties during long periods of operation involving a very
large number of charge/discharge cycles). Thermal energy storage in soil has
some additional advantages such as: favourable thermophysical properties of
the soil, availability of the SSM, minimum costs for the storage system set up
(mere drilling and tube positioning costs), and practically unlimited storage
capacity (by increasing the number of boreholes). The main drawback derives
from the TES set up: it cannot be insulated and therefore heat losses are high.
A borehole thermal energy storage (BTES) is a set of vertical tubes
inserted in the soil in order to transfer heat to and to extract heat from the SSM
represented by the soil itself. Basically, such thermal storage systems are best
suited for seasonal (long term) storage, i.e. summer/winter, but short term
storage (on a daily basis) may also be considered.
The charge/discharge rates of the BTES can be assessed by determining
the dynamics of the temperature field within the SSM. Therefore, different
approaches can be considered involving the analytical treatment of the transient
heat conduction or the numerical approach using the finite difference method
that avoids the almost insurmountable mathematical difficulties of the analytical
method.